The name of schalstein has been used, or abused, for many other kinds of rock, and hence we find a tolerably rich literature on the subject. References. Stifft, in v. Leonhard's Zeitschr. f. Min. 1825, vol. i. pp. 147 and Oppermann, Dissert. über den Schalstein und Kalktrapp, 1836. Eglinger, Analyses, in the Jahrbuch des Ver. f. Naturk. in Murchison and Sandberger, Transact. of the Geol. Soc., second v. Dechen, in Nöggerath's Rheinland Westphalen, 1822, vol. ii. p. 71; and in Archiv f. Miner. Geogn. &c., vol. xix. p. 516. Hausmann, on the Formation of the Harz Mountains, 1842, p. 23. Sandberger, Ubers. der geol. Verh. des Herzogth. Nassau, 1847, p. 33. Gumprecht, in v. L. u. Br. Jahrb. 1842, p. 825. Naumann, Erläuter. d. geogn. Karte v. Sachsen, 1836, No. 1, p. 60. Appendix. We shall here append a rock of somewhat doubtful character. LATERITE. This is the name given by English geologists to certain rocks of East India, which in part are red traps, very much resembling brick, but others are the products of the decomposition of crystalline schists. Upon such uncertain data, of course, no definite character can be established for a rock. References. Gumprecht's Zeitschr. f. Erdkunde, vol. v. p. 160. According to v. Richthofen, the laterite of Ceylon is decomposed calcareous gneiss: v. Leonhard's Jahrb. 1862, p. 739. CHAPTER IV. ROCKS OF SPECIAL CHARACTER OR BEDDING. WE propose under this general head to gather together several formations of very various character, but subordinate extent-in point of comparative bulk hardly important enough to be considered altogether essential ingredients of the earth's crust. Several of the rocks we have classed under previous heads are likewise comparatively insignificant in point of their extent, but they form part of larger connected groups, and so enter into the family of the great rock formations of the globe. In this chapter we have to deal with more separate and disconnected formations, frequently of local character only, and which we rather force into groups for the sake of convenience than in conformity with the nature of their origin, which is very various and in many cases doubtful. Some are of igneous, some of sedimentary or metamorphic origin, but others, in their bedding and composition, differ so much from the greater part of the rocks of each of those three classes, that we are compelled to regard them, for the present at least, as problematical formations, although we may account for several by supposing a concurrence of extraordinary and exceptional circumstances at their first origin or during their mutations. We have not, therefore, attempted to classify these special rocks according to origin; but have arranged them somewhat arbitrarily in groups in the following order : 1. Serpentine rocks. 2. Garnet rocks. 3. Greisen and schorl rocks. 4. Coal and carbonaceous rocks. 5. Ironstone rocks. 6. Various minerals as rocks. SERPENTINE GROUP. These are rocks, probably, of very various original character, but which have all undergone the same special transmutation. This process has not been one of increase of crystallisation, nor of actual decomposition: it seems to have simply consisted in the absorption of magnesia, just as we know has happened in the case of many and various minerals. These have been converted from their original state into serpentine, steatite, or other magnesian compounds, and are pseudomorphs retaining the form of their original crystallisation. 44. SERPENTINE, OPHIOLITE. SERPENTIN, OPHIOLITH. (Germ.) A compact rock, dull in fresh fracture, soft, with greasy feel, usually dark-green or brown. It may be doubted whether serpentine exists as an original and independent mineral; for the crystals with amorphous fracture, which some mineralogists call serpentine, according to others are nothing more than pseudomorphs of chrysolite or some other mineral. If, however, the existence of serpentine as an independent mineral were established, the question still remains whether the rock which we term serpentine is to be regarded as consisting of such mineral, because, although its composition is similar, in many cases it may be distinctly shown that the rock has been derived by transmutation from other rocks. We know of undoubted pseudomorphs of hornblende, felspar, augite, &c., consisting of a substance bearing at least a very close resemblance to serpentine, and actually so called. We will not pursue this mineralogical question further, but proceed to the description of the rock. Serpentine rock consists of two-thirds silicate of magnesia combined with 12-21 per cent. of water. It also contains some protoxide of iron, and this, as well as the water, enters into combination with the silica, supplanting a part of the magnesia: the proportion of silica varies from 38 to 43 per cent.; the magnesia from 34 to 44; lime, clay, manganese, bitumen, and carbon are only present in small quantity. The mass is so soft and tractable, and yet so tough, that it admits of being cut into various shapes or turned with the lathe. Its unctuous feel is a very characteristic property of serpentine, and is caused by the great quantity of magnesia which it contains. Probably the numerous friction surfaces which often divide the rock in all directions are also owing to the presence of magnesia. These surfaces have a resinous lustre and are sometimes striped. The rock is usually of a dark-green colour, but some varieties are light-green, grey-green, brown, reddish-brown, or almost black, and the rock sometimes presents rapid alternation of colour, causing spots, flames, or vein-like markings. The principal mass of serpentine often porphyritically encloses many minerals of various kinds. The most frequent are pyrope, or magnesia-garnet, sometimes accompanied by talc, less frequently bronzite, schiller-spar, chlorite, mica, magnetic iron-ore, pyrites, mispickel, chromic iron-ore, and very rarely (in the Ural) native platinum. The quantity of magnetic iron-ore is exceptionally so considerable, as to influence the magnetic needle; for instance, in the Fichtelgebirge, where, however, the rock is not a very characteristic serpentine. The mass of serpentine rock is frequently penetrated by veins consisting of fibrous serpentine (asbestus), chrysotile, chlorite, or picrolite. Somewhat more rarely there occur veins or nests of calcspar, calcareous magnesian spar, magnesite, saponite, pyknotrope, dermatine, talc, brucite, völknerite, hornblende, strahlstein, quartz, chalcedony, jasper, chrysoprase, opal, pyrites, chalcopyrite, chromic iron-ore, magnetic iron-ore, and native copper. Inasmuch as all serpentine is probably the product of the metamorphosis of some other rock, it need hardly be said that transition states of this metamorphosis are found which differ not only from the extreme result of the process of change the genuine serpentine-but from each other. If, however, this theory of the origin of serpentine be well founded, we cannot always succeed in determining with certainty the character of the original rock; perhaps in these cases the whole of the rock's mass has undergone change, and if bordered by other rocks of a different character, no trace is left of its original composition. Several of the transition states of serpentine have received specific names. (e) FORELLENSTEIN (Germ.) or TROUT-STONE, at Neurode, in Silesia. A compact labradorite mass, speckled with spots of serpentine, which are frequently of angular form, and which Von Rath believes to have formerly been crystals of labradorite now converted into serpentine. (f) RENSLAERITE is the name given by Emmons, in his American Geology, 1855, to a serpentine-like rock, somewhat more crystalline than ordinary serpentine. Its colour ranges from greyish white to green or black. Specific gravity, 2.87; composition, 59.2 silica, 32.9 magnesia, 34 protoxide of iron, 1 lime, and only 2.8 water. (g) SCHILLER ROCK. SCHILLERFELS. (Germ.) The name given to a compound of schillerspar and serpentine, which goes over into ordinary serpentine. It occurs at the Baste in the Hartz Mountains. It has a serpentine matrix enclosing crystals of schillerspar of considerable size. It also contains labradorite, augite, mica, chlorite, and pyrites. Cocchi proposes that serpentine rocks should be designated according to the particular rocks from which they sprang; e.g. diallage-serpentine, diorite-serpentine, granite-serpentine, &c. This may be very advisable where it is possible. Serpentine for the most part is jointed into irregular, massive, or gnarled masses. Exceptionally it is of columnar structure, but not unfrequently it shows a kind of stratification or tabular jointing. This latter may have been occasioned by actual stratification, since serpentine may well have arisen from stratified rocks. It is most frequently found in irregular and subordinate beds between strata of crystalline schist, but it also occurs in |